DeWalt Transmission questions

[StevenB]

First, I want to thank Dr. Joe, Joe P., and all of the others that orginally figured out how to do this. We're currently trying to use to CIMs with the DeWalt transmission, and have a bunch of questions.
1) We bought the drills whole, and can't get the chuck off of the output shaft. We removed the left-hand screw at the back of the chuck, but the pieces aren't coming apart. How have other teams buying the whole drills done this?
2) Why was it chosen to remove the first stage of the gearing in the CIM design? With an appropriate adapter, would it work to leave the first stage in? Would this going to result in broken gears?
3) Other than weight, is there any reason the tophat is plastic? Will anything that sticks out far enough be sufficient to hold the shifting ring in place?

We're looking at making an adapter to hold the sun gear on the shaft and avoid drilling out the sun gear entirely. Has anyone done this? I'll post a picture soon.
Thanks!

[Jack Jones]

Originally Posted by StevenB

We're looking at making an adapter to hold the sun gear on the shaft and avoid drilling out the sun gear entirely. Has anyone done this? I'll post a picture soon.
Thanks!

Do you mean something like this?
First stage still goes away because adapter replaces the planets. MAX output is about 480 RPM in low gear with CIM.

[Ben Piecuch]

Answers:

1) We had to use a 3-4 ft lever arm to remove the chuck from the rest of the gearbox. IIRC, we chucked up a large hex bolt, grabbed a wrench and a really long lever arm, and started turning. It eventually broke loose. I think we had to hold the entire gearbox and housing in a vice. (I know we took pictures, they're just not up on our website...)

2) The OEM motor for that gearbox spins between 15-20,000 rpm's. That extra planetary stage is needed to reduce the output speed to something usable on the drill. The CIM only spins around 5,000 rpm's, therefore the removed stage to achieve a similar output rpm. If your application requires that 1st stage in the gearbox, I'm afraid you'll have to engineer that yourself.

3) I believe you are correct. The tophat is plastic because it's easier and cheaper to machine it out of plastic. I don't think it really takes any stress at all.

BEN

[Joe Johnson]

There is absolutely no reason that the tophat was made from plastic other than we had a blank of plastic laying around.

In actual fact, I think the part is too complex. It would be much better to just make a flat washer with the ID of the motor shaft and the OD of the Tophat. Use washers to space it out from this washer back from the sun/carrier (though it is not a carrier any more since we've taken out stage 1). Use a simple clamp collar or another spacer to keep it the right distance away from the motor.

The only reason the tophat is needed is to keep the ring from tilting when it is shifted toward the motor (i.e. locking gear stage 2 - the ring, sun and planets all rotate in unison).

I just don't have time right now, or I would mock this up in CAD and make an improvement to the NBD paper.


Joe J.

[StevenB]

Attached is a drawing of what we were planning. This should eliminate drilling the sun gear completely, but we will have to push the CIM back by the height of the sun/carrier.
We did some rough calculations, and it seems that even with the CIM being slower, leaving the first stage in would mostly eliminate the need for a large external sprocket ratio. Was it simply for speed that the first stage was taken out, or are there other problems?

[Joe Johnson]

Originally Posted by StevenB

Attached is a drawing of what we were planning. This should eliminate drilling the sun gear completely, but we will have to push the CIM back by the height of the sun/carrier.
We did some rough calculations, and it seems that even with the CIM being slower, leaving the first stage in would mostly eliminate the need for a large external sprocket ratio. Was it simply for speed that the first stage was taken out, or are there other problems?

As to your mod to eliminate the machining of the sun/carrier, I think I like it! The only issue is the length of the whole assembly, but other than that, it is a nice solution.

As to taking out the 1st stage or leaving it in. The main reason I recommend taking it out involves chain force. In theory, if you leave the 1st stage in and then reduce the chain stage by the same ratio, all is the same, but.... ...not really. By putting the stage in the tention in the chain goes up by a factor of about 3. This make a huge difference because that means the side loading on the gearbox is 3X and as well as the side loading on the wheels from the chain.

If you are really brave, you may consider leaving in the gearstage and eliminating the chain stage entirely by going direct drive right to the wheel. I've made prototypes chassis this way, but I was never a competition robot.

Joe J.

[Ben Piecuch]

I've made prototypes chassis this way, but I was never a competition robot

Joe, you'll always be a competition robot to me!

Steven, I looked quickly at your drawing and agree that it is a much nicer solution. However, I haven't looked at how it actually interfaces with the rest of the transmission. Meaning, does the CIM adapter plate change? Do you have to make any further modifications to the DeWalt housing? Also, make sure you do a stress calculation on the key engagement, with respect to the new gear/carrier assembly. (Though I agree that it should be on the low side...)

We didn't have any issues with side loading with our setup. We used the suggested C-Clamps in the whitepaper, but swapped out the steel ones for aluminum ones to save some weight. Again, this was with the "standard" setup, not one you are suggesting. If you can engineer a way to fit two sprockets on the end of the output shaft, I'd be happy to hear about it. That would cancel out the side loading on the shaft, as well as make 4wd and 6wd setups a lot easier to run chain to.

Keep up the good work,

BEN

[groves]

This is the connection we have been using four a couple years now. we have used this with direct drive and we have been extremely happy with this system. The key is built into our disk (2 years with zero failure). The motor slides into the disk without a set screw (where is it gonna go?)

you can search for 1213 in the images and see more pictures of our system...

[Andy Brockway]

Originally Posted by Ben Piecuch

....... If you can engineer a way to fit two sprockets on the end of the output shaft, I'd be happy to hear about it. That would cancel out the side loading on the shaft, as well as make 4wd and 6wd setups a lot easier to run chain to.

Keep up the good work,

BEN

I did this a couple of years ago for #25 chain here .

I have refined it a bit and will see if I have a picture to upload.

[Joe Johnson]

Originally Posted by Ben Piecuch

J...If you can engineer a way to fit two sprockets on the end of the output shaft, I'd be happy to hear about it. That would cancel out the side loading on the shaft...

At first blush this seems to be true but in practice it is not. Basically, the load on the tires are not uniform so the traction forces are not equal.

Think of it this way, imagine a 4WD robot with its CG in the middle pushing against a wall. As the robot slowly hits the wall, the load on the tires is equal but the harder the robot pushes on the wall, the more load shifts to the rear wheels until the front wheels are effectively off the ground and the rear wheels take all the load.

There are good reason to put 2 sprockets on the output shaft of the drive more, but I think that cancelling out the side load on the shaft is not as strong of a reason as one might think.

Joe J.

[Ben Piecuch]

Joe, I understand how the forces will vary as the drivetrain is put through a "real world" test. The true advantage is that your chain runs are much easier to deal with when having two output gears. We ran one piece of continuous #35 chain last year, as we only had 1 DeWalt setup per side. It caused us plenty of problems, and having the ability to run two seperate chains would have made our drivetrain a lot more robust.

So, with a greater amount of teams starting to use, and improve these DeWalt trannies, how long until we start to see some updated Whitepapers? Also, who's going to step up to the plate and become an official "supplier" of the modified DeWalt setup. That way, we can reuse these beauties year after year.

BEN

[StevenB]

1) We had to use a 3-4 ft lever arm to remove the chuck from the rest of the gearbox. IIRC, we chucked up a large hex bolt, grabbed a wrench and a really long lever arm, and started turning. It eventually broke loose.

The chuck/gearbox shaft isn't threaded, correct? I attempted the long lever trick this morning, and broke a Craftsman ratchet. I'll have to try again with a regular wrench or a stronger ratchet.

we have used this with direct drive and we have been extremely happy with this system.

What size wheels are you using? Do you have any other gearing? What speed does this make the robot go?
We have thought about this, as it would elimate a lot of weight, as well as some of the problems with chain and sprockets.

[Tristan Lall]

Originally Posted by StevenB

The chuck/gearbox shaft isn't threaded, correct? I attempted the long lever trick this morning, and broke a Craftsman ratchet. I'll have to try again with a regular wrench or a stronger ratchet.

The gearbox outputs to a shaft that is not only right-hand threaded on the outside, but left-hand threaded on the inside. Did you remove the locking screw from the left-hand threads?

Also, Craftsman hand tools tend to have a lifetime warranty. Check into it—you usually won't need a receipt, as long as the name "Craftsman" is visible.

[Ben Piecuch]

The chuck is threaded to the gearbox, a right hand thread. The left hand thread was only for the retaining screw with the torx head. (Tristan beat me to it...)

The pictures I found only show us clamping the entire drill assembly to the table. I couldn't find anything that showed our lever arm. But, I'm not sure what kind of wrench you broke, or how. The chuck can accommodate a 1/2" bolt, so I would suggest chucking up a bolt that size, getting the biggest wrench you can on the head of the bolt, and go to town. When it breaks free, you'll know.

Heat can also be your friend here. If you have a small torch, you can heat up the outside of the chuck, and be very careful not to heat the gearbox, the housing, etc... But, I would only go this route as a last ditch effort.

Team 1213 used some pretty small wheels, 4", which gave them about 5-6' per second in low gear. I'm not sure that they actually shifted the DeWalts. If they did, they'd have a theoretical top speed in the 24fps range. Anyone care to comment on this, or did I just make up a rumor?

BEN

[groves]

What size wheels are you using? Do you have any other gearing? What speed does this make the robot go?

Team 1213 has used 4 inch wheels directly driven during the regular season the last two years (giving us about 8 fps using the lowest gearing) This off-season we used 5 inch (11 fps). This season we are working on a 13 fps system using a 6 inch wheel. One of the best places for robot parts is NPC and we get out wheels from them (1/2 inch keyed hubs). This is where we get other things like the dewalt trannies.

We use only the dewalt transmission to gear our cims down. The output theoretically is 555 rpm's. Here are some pictures of our drivetrain all assembled and in parts.

Team 1213 used some pretty small wheels, 4", which gave them about 5-6' per second in low gear. I'm not sure that they actually shifted the DeWalts. If they did, they'd have a theoretical top speed in the 24fps range. Anyone care to comment on this, or did I just make up a rumor?

We have shifted gears, however we never decided to use it during game play.



What would be the reason for using the dewalt output shaft? We have used 1/2 inch steel shaft modified to match the octagon shaped output on the transmission. Now we are testing a new star disk that replaces the octagon shaped disk on the transmission. Our new disk matches the 1/2 inch steel shaft the key is included in our star disk. (the reason for this "star disk" is that we sheered the end of the modified shaft once in two years. that is the only failure we have had yet) Jack Jones our teams engineer is the designer of this drive system. I'm sure he would be glad to share more information if you would like to hear more (he is very helpful).

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